1. Technical Field of the Invention
The present invention relates generally to an automotive ac generator for use in passenger vehicles and trucks, and more particularly to an ac generator designed to minimize vibrations of coils during operation.
2. Background Art
Recently, there are increasing needs for high power of ac generators for automotive vehicles involved in increasing electrical load consumed by safety control devices etc. and for reduction in magnetic noise produced by the ac generators for meeting environmental requirements to decrease noise outside the vehicles and improvement of silence in a vehicle cabin.
International Patent Publication No. WO98/54823 discloses an automotive ac generator which is designed to increase an output power by connecting ends of stator windings of a coil using a plurality of conductive segments in the same pattern in a circumferential direction of the coil to eliminate the interference between the stator windings and to improve the space factor and the cooling of the stator windings. Additionally, the ac generator is also designed to combine outputs from two groups of windings different in phase to reduce magnetic noises.
International Patent Publication No. WO92/06527 discloses a manufacturing method of a stator using a plurality of conductive segments.
Usually, the generation of magnetic noises is caused by magnetic pulsation in a gap between a stator and a rotor of an ac generator which produces vibrations of a core of the stator which are, in turn, transmitted to a frame and its brackets etc. mounted on the side of an engine body and then radiated as compressional waves of air. Additionally, when the natural frequency of ends of coils wound in slots of the stator extending in an axial direction of the stator agrees with the frequency of the magnetic pulsation, it will cause the ends of the coils to resonate, thereby resulting in an increase in radiation of magnetic sound from the ends of the coils.
Particularly, most of modern automotive ac generators have internal cooling fans and many ventilating windows formed in a frame facing ends of coils. The magnetic noises are, thus, emitted directly outside the ventilating windows without being blocked by the frame. Additionally, when the ends of the coils are resonating, a vibrational component of the resonation in a circumferential direction of the coils acts to rotate a stator core retained by the frame in an axial direction of the coils. If, therefore, the resonation of the coil end is repeated, it will cause the stator core to rub against an inner wall of the frame, resulting in excessive wear of the frame because the frame is usually made of a light weight and non-magnetic material such as aluminum. The wear of the frame will causes a fixative power serving to hold the stator core in an axial direction thereof to be lowered, thereby resulting in an increase in vibration of the stator core induced by the magnetic pulsation, leading to an increase in magnetic noise. Even when the natural frequency of the ends of the coils agrees an integral multiple of (more than two times) the frequency of the magnetic pulsation, the circumferential vibrations of the coil ends are not produced as much as when resonating, but increase.
International Patent Publication No. WO98/54823, as discussed above, fails to teach how to reduce the circumferential vibrations of the coil ends. The structure, as disclosed in International Patent Publication No. WO92/06527 has two conductive segments arrayed in each slot in the same radial direction. An increase in vibration in the circumferential direction of coil ends, thus, cause the conductive segments to rub on each other, which may result in damage of insulating coatings on surfaces of the conductive segments, thereby short-circuiting the conductive segments.
The above described International Patent Publication Nos. WO98/54823 and WO92/06527 do not refer to the resonance frequency of the coil ends of the automotive ac generators at all. The present invention takes it into consideration and provides an idea that an increased degree of rigidity is added to ends of coils of an ac generator to shift the resonance frequency outside a normal speed range of the ac generator.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide an ac generator designed to minimize resonance of coil ends in a circumferential direction thereof to reduce magnetic noises.
It is a further object of the invention to provide an ac generator designed to avoid electrical short of coil ends caused by vibrations of the coil ends during operation.
According to one aspect of the invention, there is provided an automotive ac generator which comprises: (a) a rotor having a magnetic core, the rotor rotating within a preselected speed range; (b) a stator including a hollow cylindrical stator core having opposed ends and a plurality of electrical conductors, the stator core surrounding a periphery of the rotor and having formed therein a plurality of slots in which the electrical conductors being disposed; (c) a pair of cup-shaped frames retaining the stator; and (d) conductive bridges projecting from the opposed ends of the stator core and connecting the electrical conductors disposed in one of the slots and the electrical conductors disposed in another slot to form stator windings, the conductive bridges being so designed as to resonate outside the preselected speed range of the rotor.
In the preferred mode of the invention, the electrical conductors disposed in each of the slots are arrayed in a radial direction. Each of the conductive bridges includes two slant sections which define two equal sides of substantially isosceles triangle and which make an angle of more than 100xc2x0 with each other.
The electrical conductors disposed in each of the slots are insulated electrically from each other and are made of at least two conductive layers: one is an outer conductive layer disposed close to an outer peripheral wall of the stator core and the other is an inner conductive layer disposed close to an inner peripheral wall of the stator core. The outer conductive layer in each of the slots is connected in series through one of the conductive bridges to the inner conductive layer in another slot to form one of the stator windings.
The outer conductive layer and the inner conductive layer which are connected to each other through one of the conductive bridges are formed by a single U-shaped conductive member having a curved portion constituting the one of the conductive bridges.